This invention relates to a system for removing pollutants from a flue gas, and more particularly to a method for forming and then removing particulate matter from a relatively dry flue gas stream.
Many industrial or commercial facilities generate and discharge hot streams of flue gas to the atmosphere. The flue gas often includes entrained solids, along with pollutants in the form of gases, mists and vapors. These pollutants must be removed before discharging the flue gas into the atmosphere, in order to prevent atmospheric pollution. Further, in many instances certain of the impurities found in the flue gas may be sufficiently valuable so as to justify their recovery.
Many diverse types of purification apparatus and methods are known in the prior art. These approaches may generally be categorized as either "wet" or "dry" methods. With dry methods, particulate matter in a flue gas stream is removed by physical separation. This is ordinarily effected through the use of filters, electrostatic precipitators, or centrifugal separators such as cyclones. While dry methods of separation have several desirable attributes, it will be recognized that they have no utility in removing vapors or gaseous contaminants from the flue gas stream.
Wet separation methods, on the other hand, can be used to remove various gaseous and vaporous impurities, as well as solid materials, from the flue gas. In particular, where acidic gaseous or vaporous impurities are present it is known to bring the flue gas into intimate contact with a basic solution or slurry (also termed a liquor).
The liquor is usually constituted of a water slurry bearing a basic reactant such as crushed limestone. By vigorously intermixing the vapor-bearing flue gas and the scrubbing liquor it is possible to react the scrubbing material with the acidic impurity to produce a neutral product. As the product is formed it is automatically carried from the flue gas stream by the entraining liquid of the slurry, hence the term "washing".
One example of a system of the latter type is U.S. Pat. No. 3,785,119--McIlvaine. McIlvaine teaches that solid, particulate material in a flue gas may be removed by a conventional dry separation stage. A wet scrubber absorber is then used to treat a fractional portion of the flue gas to allow a gaseous impurity (i.e., sulfur dioxide) to be absorbed directly into a water slurry of limestone. The wet, sloppy precipitate which is then formed is transferred from the presence of the gas. The washing or scrubbing is produced by a spray of finely-divided slurry discharged counter to the flow of flue gas, collecting at the bottom of the scrubber housing from whence it is pumped to a holding tank. The washing of hot flue gases with wet precipitates in conventional processes saturates the flue gas with water and produces a wet, pasty mass which cakes the surrounding surfaces of the ducting and flue gas processing apparatus. Still further, the composition of the caked slurry is such as to attach ordinary mild steel, galvanized surfaces and the like so that the portions of the ducting and the processing apparatus which come in contact with the flue gas must be formed of stainless steel, rubber-lined steel or similar expensive corrosion-resistant material. To avoid these plugging difficulties many systems employ "two stage" scrubbing using sodium solutions (NaOH or Na.sub.2 CO.sub.3) in the scrubber and then employing a second reaction in a liquid waste treatment plant, exchanging calcium from lime or the sodium used in the first step. This system is quite expensive.
Another deficiency of prior art methods of washing or scrubbing flue gases is that the resulting slurry is relatively difficult to handle. The particulate-bearing slurry is collected at an appropriate point in the processing system, in a sump or the like. The wet, sloppy mass must then be pumped to a remote site for disposition, or to a settling tank followed by a vacuum filter or centrifuge or the like where water and/or divided material from the slurry are separated and treated for recycling. Accordingly, it will be recognized that it would be highly advantageous to provide an improved system for removing vaporous and gaseous contaminants from a flue gas stream without the production of a wet, sloppy precipitate including a reaction product.
It is therefore an object of the present invention to provide an improved method for treating flue gases.
It is another object to provide a method for removal of vaporous or gaseous contaminants from a flue gas.
Yet another object is to provide an improved method for removing acidic contaminants from an industrial flue gas.